1. Field of the Invention
The present invention relates to a system for detecting the pneumatic pressure in a tire for vehicles.
2. Description of the Prior Art
Known systems for detecting the pneumatic pressure in tires are disclosed in Japanese Patent Publications Nos. 51-9639 and 56-10202, for example.
According to the tire pressure detecting system disclosed in the former publication, when the pneumatic pressure in a tire on a vehicle drops below a certain predetermined pressure level while the vehicle is running, a lock member which has engaged a shunt block under the tire pressure is displaced out of locking engagement with the shunt block. The shunt block is now turned away from a magnet under centrifugal forces produced by the tire, and the magnetic flux from the magnet turns on a reed switch to produce a warning signal.
The tire pressure detecting system disclosed in the latter publication has a plunger which is normally biased in locking engagement with a shaft under the tire pressure, the shaft being coupled to a magnet. When the pneumatic pressure in a tire on a vehicle drops below a certain predetermined pressure level while the vehicle is running, the plunger is displaced out of locking engagement with the shaft, and the magnet is turned under the bias of a spring, causing a reed switch to produce a warning signal. When the vehicle comes to a stop, the magnet is automatically returned to an initial position by a spring-biased actuator which has been angularly displaced under centrifugal forces produced by the tire during rotation thereof.
Therefore, the above known tire pressure detecting system can detect when the pneumatic pressure in the tire has dropped below a predetermined pressure level.
Another tire pressure detecting device disclosed in Japanese Laid-Open Utility Model Publication No. 59-146748 serves to linearly detect the pneumatic pressure in a tire. The disclosed tire pressure detecting device has a spring which expands or contracts depending on the pneumatic pressure in a tire. The pneumatic pressure in the tire is linearly detected depending on the position of a magnet which is angularly movable as the spring is displaced.
The tire pressure detecting systems disclosed in Japanese Patent Publications Nos. 51-9639 and 56-10202 are not capable of linearly detecting the pneumatic pressure in a tire. Furthermore, since the shunt block is angularly moved under centrifugal forces during rotation of the tire, the pneumatic pressure in the tire cannot be detected when the vehicle is at rest in Japanese Patent Publication No. 51-9639. In Japanese Patent Publication No. 56-10202, the magnet is not automatically returned to the initial position if the pneumatic pressure in the tire drops while the vehicle is held at rest. The tire pressure detecting device shown in Japanese Laid-Open Utility Model Publication No. 59-146748 cannot effect accurate detection of the tire pressure because the spring and the magnet are also subject to centrifugal forces produced by the tire.